Everything in politics has an arc — a beginning, a high point and an end. The art of campaigning is to hit the high end of that arc as close to Election Day as possible.

That happened in 1980 when Ronald Reagan, after stumbling for two weeks following the Republican convention in Detroit, regained his footing, restored his momentum and won 41 states on Election Day. In 1988, Massachusetts Gov. Michael Dukakis ran 17 points ahead of Vice President George Bush — in July. From then on, everything ran downhill and Dukakis won just 10 states. On the evening before Election Day 2000, Dick Cheney told me a network poll had George W. Bush up 6 points. But the momentum had been moving toward Al Gore. Had the election lasted 24 hours longer, Gore probably would have peaked at just the right time.

Barack Obama has generated more excitement this year than any presidential contender in at least a generation. Having seen nothing like him in their lives, young people have signed up in droves. Older Democrats say the last candidate who connected with them this way was Bobby Kennedy in '68. Women faint at his rallies. That wouldn't happen at a John McCain or Hillary Clinton event unless it was held in 110-degree heat.

But excitement is closely tied to momentum and the Obama campaign is losing both. The affection for him is genuine, but it's less a long-term romance than a crush. And everyone knows that crushes either crash or fade. Ask an Obama supporter about the senator's greatest political accomplishment and the reaction is often the same: a crinkled eyebrow, an awkward acknowledgment that they can't think of anything, but he still inspires them because he represents "change" and "hope."

OK. But soaring, uplifting sermons promising "hope" and "change" eventually run dry unless they're connected to clear ideas and a coherent agenda. Martin Luther King's "I have a Dream" speech was about ending segregation in the pursuit of racial equality in every aspect of life. He was speaking truth to power for a clear purpose.

But, Obama's words aren't a bridge to ideas and opinions, they're substitutes for them. He calls for common ground, but the senator actually has a more liberal voting record than Hillary Clinton and is much more ideological and partisan in the Senate than McCain.

Obama's losses in both Texas and Ohio underscore why time is not on his side. These were the first primaries that didn't follow on the heels of another with another contest immediately following. Instead voters were able to sit back for three full weeks, listen to the debates, watch how the candidates and their spouses talked to different audiences in different parts of the state, hear their advertising and take their time digesting this information and discussing it with others at home, work and the barber shop.

When they did that, Obama began to fade. Like a hit record that's been on the charts for a while, they still smile when it plays but they're getting used to hearing it. In Ohio, a must-win state for the Democrats in November, people began to tire of it. Isn't there a "B" side?

Most Americans like Obama but they don't know him, and liking and trusting aren't quite the same thing. A TV spot asking whom voters would rather have picking up the phone at the White House during an overseas crisis at 3 a.m. simply asked what any reasonable voter would consider before pulling the lever in November. That's hardly a low blow or an act of "desperation" by the Clinton people. (If the McCain campaign is smart, it'll rerun that ad in the fall, with McCain picking up the line.)

And Michelle Obama didn't help with her comment about finally, in her 40s, "being proud of my country for the first time," and suggesting to a young audience in a working-class Ohio town that they should sidestep "corporate America" and instead seek out more rewarding, lower-paying jobs in teaching and social work. Whom did she think she was talking to, the senior class at Vassar?

There is much to like and admire about the first post-'60s candidate for president. But his constant incantations of "change" aren't enough — especially when your Democratic opponent and Republican challenger already offer a clear change from the status quo.

The senator has built up a huge wave of momentum and he is still the odds-on favorite to get the nomination. But even as he surfs, the wave is beginning to crest.

John Carlson is a news commentator for KOMO-AM (1000), and talk-show host with Ken Schram on "The Commentators," from 3 to 6 p.m. weekdays on KVI-AM (570).